Knee injuries

From Medical-Wiki

The knee is perhaps the most mechanically unstable joint in the body. Its stability is related to very complex configurations of ligaments, fascial layers, and tendon insertions. This means that to a great degree, the stability is determined by the muscles that act on the joint—particularly the quadriceps muscle.

As a consequence of this anatomy, injuries run the gamut from single element injuries to multiple element injuries to a complete loss of function.

Orientation to the anatomy of the knee is best done visually. [1]

Contents 1 Motions & the tendency for injury 2 Diagnosis 2.1 ACL 2.2 PCL 2.3 Knee rules 3 Treatment

Motions & the tendency for injury

The motions that are most likely to injure the knee are twisting and stretching—these carry the joint through a greater range of motion than it can tolerate. As the knee is stressed, the ligament holding it in place is stressed as well. Ligament stretching or partial tears are called sprains. These are graded as grade one—stretch of the ligamentous but no tear; grade two—partial tear of the fibers but ligament intact; and grade three—complete tear of the ligament.

If the force applied to the knee is a twisting motion, the meniscus can be pinched between the tibial surface and the femoral condyle. Hyperextension and hyperflexion can cause small tears in the muscles and tendons that act on the knee. These are called strains. [2]

Valgus stress without a rotary component will cause an isolated medial collateral ligament (MCL) injury. However, MCL injuries rarely occur by themselves. Typically they are accompanied by anterior cruciate (ACL) tears and medial meniscus tears.

Varus stress against a knee that is internally rotated will produce an isolated lateral collateral ligament (LCL) injury. Isolated injuries of the LCL are also uncommon and usually occur in association with ACL or posterior cruciate ligament (PCL) tears. [3]

Direct trauma or a powerful contraction of the quadriceps may fracture the patella. Patellar fractures are divided into transverse, vertical, upper pole, lower pole, comminuted and osteochondral fractures—undisplaced or displaced.

Fractures of the femoral condyles usually occur as the result of auto accidents. They may extend into the joint. In older patients, osteoporosis makes this injury more likely.

The tibial plateau is a crucial weight bearing surface. Fractures of the plateau may involve the metaphysis, epiphysis and/or articular cartilage. Typically, fractures are of the lateral condyle or both the medial and lateral—bicondylar fractures.

Diagnosis

Plain radiographs have a high rate of sensitivity, but there are specific views used to identify patellar fractures.

ACL

Complete rupture of the ACL is often accompanied by an instant and significant hemarthrosis. When it’s not, the anterior drawer sign and Lachman test are a time honored means of initially testing the integrity of the ACL. However, it is well established that these have a very low rate of sensitivity. An injured knee is better at guarding than even the strongest examiner is at testing its stability. For this reason, rupture of the ACL is often missed. It is incumbent upon all practitioners to appropriately evaluate the possibility of an ACL injury. In this regard, if arthroscopy is the gold standard, MRI has a sensitivity of 90%.

PCL

PCL injuries are relatively uncommon. There are clinical and radiographic signs of its rupture, but once again, MRI should be used to appropriately assess the ligaments integrity. The same may be said for MCL, anterior capsule, and meniscus injuries.

Knee rules

This having been said, not every knee injury needs an MRI. Because knee injuries are exceedingly common, it has been suggested that not even every knee injury needs an x-ray. A set of criteria called the Ottawa Rules have been designed to suggest when x-rays were needed. In 1996, a prospective study evaluated the Ottawa Knee Rules. “The study found that the decision rules were 100 percent sensitive for identifying knee fractures, were reliable and acceptable, and had the potential to allow physicians to reduce the use of radiography in patients with acute knee injuries.” [4]

The Pittsburg Knee Rules were developed at the same time as the Ottawa Knee Rules. A large study compared the two and concluded that the Pittsburg rules were as sensitive and more specific than the Ottawa rules and have become preferred—except perhaps in Ottawa. Those rules are: blunt trauma or a fall as mechanism of injury plus either of the following, age younger than 12 years or older than 50 years, and/or inability to walk four weight-bearing steps in the emergency department. [5]

Treatment

Most knee fractures require internal fixation. Consequently, immediate orthopedic referral is appropriate.